In a fuel cell a fuel, which is typically hydrogen or an alcohol, such as methanol or ethanol, is oxidised at a fuel electrode (anode) and oxygen, typically from air, is reduced at an oxygen electrode (cathode) to produce an electric current and form product water. An electrolyte is required which is in contact with both electrodes and which may be alkaline or acidic, liquid or solid. The liquid electrolyte phosphoric acid fuel cells operating at temperatures of 150° C.-210° C., were the first fuel cells to be commercialised and find application in the multi-megawatt utility power generation market and also in combined heat and power i.e. cogeneration systems, in the 50 to several hundred kilowatt range. More recently, fuel cells in which a phosphoric acid-doped polybenzimidazole membrane is used as the electrolyte have been utilised for power generation, typically in the 1-5 KW range, at temperatures in excess of 120° C.
To assist the oxidation and reduction reactions that take place at the anode and the cathode, catalysts are used. Precious metals, and in particular platinum, have been found to be the most efficient and stable electrocatalyst for fuel cells operating at temperatures below 300° C. The platinum electrocatalyst is typically provided as very small particles (˜2-5 nm) of high surface area, which are often, but not always, distributed on and supported by larger macroscopic electrically conductive particles to provide a desired catalyst loading. Conducting carbons are typically the preferred material to support the catalyst.
For use in a phosphoric acid fuel cell, hydrogen-rich fuel gas is obtained by external reforming of hydrocarbons, such as natural gas. Such a process produces, in addition to hydrogen, a mixture of gases including carbon monoxide at a level of 1-2%. Carbon monoxide is known to poison a pure platinum catalyst, even at relatively low levels and at the temperatures at which the phosphoric acid fuel cell is operated. Thus, researchers have been investigating ways of reducing the carbon monoxide content of hydrogen fuel before it enters a fuel cell as a possible way to avoid poisoning the catalysts. However, extensive carbon monoxide reduction or clean-up processes invariably increase the size, complexity and cost of the fuel reformer system, often to prohibitive levels. An alternative simpler and more cost-effective solution is to provide a catalyst that is itself intrinsically more tolerant to carbon monoxide and achieving this has also been the subject matter of considerable research.
U.S. Pat. No. 5,183,713 discloses a platinum-tantalum alloyed supported catalyst comprising 2 to 50 weight % platinum deposited on a support. The platinum-tantalum alloyed catalyst comprises about 2 to about 10 atomic % tantalum. The preferred range of tantalum in the catalyst is between about 5 and 8 atomic %. The tantalum, at percentages above about 8 atomic %, covers part of the platinum surface thereby interfering with fuel reaction sites, while at percentages below about 2 atomic % it causes a decrease in the carbon monoxide tolerance to unacceptable levels.